Field of the Invention
The present disclosure relates to a method and apparatus for cooling. The present disclosure relates more particularly to a method and apparatus for high performance cooling.
Description of Related Art
Various methods to manufacture thermoplastic blown films are well known in the plastics art, and typically involve forming a continuous, vertically oriented, seamless, annular plastic film commonly referred to as the “tube” or “bubble”. Thermoplastic material is melted and pumped by an extruder through a blown film die (die), exiting as an annular flow of a molten film, continuously drawn upward by a pair of driven squeeze rollers. Gas is typically manually injected through the die to the interior of the exiting annular flow of molten film. The driven squeeze rollers act to prevent gas from escaping, trapping the injected gas inside, forming a molten film tube which is inflated by the injected gas until at the desired size and the die is sealed off to prevent inflation gas from escaping. The molten film tube is pulled upward by the driven squeeze rollers, flowing generally upward from the die through a cooling system, where it stretches, expands, and cools around the now trapped column of injected gas until it solidifies at a frost line into a solidified film tube. The solidified film tube passes through various stabilizers and enters a flattening device, which converts the tube into a flattened double thickness thermoplastic sheet of film known as “lay-flat”. The lay-flat passes through the driven squeeze rollers, and is conveyed to downstream converting equipment such as winders and bag making machines for further processing.
To remain competitive, manufacturers of blown film must maximize throughput rate and quality, however cooling system performance is a significant limiting factor. The weight of thermoplastic that is extruded per unit time divided by the circumference of the die exit, provides a commonly used measure of throughput performance, and is expressed in units of PPH/Inch, Pounds Per Hour per Inch of die exit circumference. Many different cooling systems have been developed and employed, both external and internal to the tube, and to varying degrees these systems have achieved commercial success.
Blown film cooling systems provide a flow of cooling gas typically external, but in many cases also internal to the molten film tube. Cooling systems are designed using well known Bernoulli and Coand{hacek over (a)} principles, and in many cases, apply the cooling gas to flow generally along the surface of the molten film tube to create holding forces on the molten film tube, providing for both stability and cooling of the molten film tube. Excessive holding forces, if present, can cause vibration, flutter, and high noise levels in the process, and can pull the molten film tube into undesirable contact with the cooling element, creating drag and causing marks and instability in the molten film tube. In other cases, cooling gas is instead applied generally against the surface of the molten film tube, typically creating turbulent cooling with repelling forces, requiring a separate means to stabilize the molten film tube.
External cooling systems, generally provide the primary means for stabilization and cooling of the molten film tube, are generally easy to operate and used on most blown film extrusion processes. External cooling systems provide a flow of cooling gas along the outside surface of the molten film tube that typically generates holding forces while cooling the molten film tube, until the cooling gas dissipates into the surrounding atmosphere. Less typically, cooling gas is aimed generally inward generating repelling forces while cooling the molten film tube, undesirably requiring a separate means to hold and stabilize the molten film tube.
Present art external cooling systems are made up of various types of cooling elements. The earliest cooling element, known as a “Single Flow air ring”, still in common use today, applies a single flow of cooling gas around the molten film tube. Single Flow cooling elements typically produce good film quality, but at lower throughput rates. Additional flows of cooling gas have been added to cooling elements over time to create various multiple flow designs, such as “Dual Flow”, “Triple Flow” or “Quad Flow” designs, and some external cooling systems pair cooling elements into various configurations, depending on the application, to form what is generically known as a “Tandem” air ring. External cooling systems are typically fixed in place, but can be made adjustable in height above the die to allow extending the cooled surface area along the molten film tube, producing higher throughput, but also resulting in greater unsupported surface area between the cooling element and die, which is the hottest and weakest portion of the molten film tube, which can lead to degraded stability, making it more difficult to operate and potentially leading to a narrower range of film sizes.
In contrast, internal cooling systems typically do not provide primary stabilization, and are selectively used typically to generate additional throughput beyond the capability of an external cooling system. Internal cooling systems replace manual gas injection and inflate the molten film tube with a flow of an internal supply gas that enters through the die. Although some recent high throughput internal cooling systems apply cooling gas to create holding forces, more typically cooling gas is directed against the inside surface of the molten film tube, acting to generally repel and cool the inside surface of the molten film tube. The flow of internal supply gas is trapped inside the bubble and cannot dissipate into the atmosphere, therefore complex control systems are used to balance a flow of internal exhaust gas that exits through the die to maintain a constant bubble size as is well known and understood by those skilled in the art. Internal cooling systems can be difficult or even impossible to use depending on such factors as operator skill, thermoplastic material properties, and the physical size and design of the associated die.
It is highly desired to overcome the drawbacks of prior art thermoplastic cooling systems and provide a cooling system that significantly increases throughput rate, maximizes aerodynamic holding forces, allows relatively large unsupported regions of the molten tube with good stability, produces a wide range of film sizes, prevents drag on the molten film surface, minimizes turbulence, vibration and flutter, does not produce high sound power levels, and is simple and easy to control.